The genesis of oxidized-type (magnetite series) and reduced-type (ilmenite series) granites is discussed on the basis of thermodynamic considerations. We show that SO 2 in the magma can oxidize augite to form magnetite with decreasing temperature because the SO 2 -H 2 S buffer line intersects with the augite-magnetite-quartz buffer line on a 1/T - f (sub O 2 ) diagram. When the magma contains SO 2 above 250 ppm and the f (sub O 2 ) is constrained by the SO 2 -H 2 S buffer, the magma forms oxidized-type granites (>0.2 modal % magnetite). On the other hand, when the SO 2 content is less than 250 ppm, the magma solidifies as a reduced-type granite (<0.2 modal % magnetite). If the f (sub O 2 ) of the magmas is buffered by H 2 O-H 2 , CO 2 -CO, or CH 4 -CO 2 no magnetite will crystallize and reduced-type granites will be formed. As a result, the genesis of oxidized-type and reduced-type granites can be attributed to the difference in volatile components in magmas.